Field
This patent application relates to an image generating apparatus, an image generating method, and a program.
Description of the Related Art
As a method for obtaining a tomographic image of an object such as an organism in a nondestructive and noninvasive manner, optical coherence tomography (hereinafter, referred to as “OCT”) has been put into use. In the OCT, in particular, a tomographic image is widely used in ophthalmic diagnosis.
In OCT, by making light reflected on a measurement object and light reflected on a reference mirror interfere with each other and analyzing the interfering light, a tomographic image of the object is obtained. As an image acquiring apparatus by optical coherence tomography, time-domain OCT in which depth information of a measurement object is obtained by changing a position of a reference mirror, Spectral Domain Optical Coherence Tomography (SD-OCT) in which interfering light is converted into a spectrum by using a broadband light source and an interfering signal is obtained, and Swept Source Optical Coherence Tomography (SS-OCT) in which an interfering signal is obtained by using a variable wavelength light source are known. Note that, SD-OCT and SS-OCT are also called FD-OCT (Fourier Domain-Optical Coherence Tomography) collectively.
In recent years, an angiography method using the FD-OCT has been proposed and is called OCT angiography.
Fluorescence imaging, which is a conventional angiography method, is invasive and requires injection of a fluorescent dye (for example, fluorescein and indocyanine green) into a body. Further, in conventional fluorescence imaging, a blood vessel serving as a passageway for the fluorescent dye is two-dimensionally displayed.
On the other hand, the OCT angiography is noninvasive because the usage of interfering light obviates the need of fluorescent dye. In addition, a signal of the OCT imaging allows three-dimensional display of a vascular network. Further, the OCT angiography is attracting attention because it has higher resolution compared to fluorescence imaging using a fundus camera and a scanning laser ophthalmoscope (SLO), and allows visualization of a microvessel of the fundus of an eye.
Various methods are proposed for the OCT angiography according to a difference of an angiography method. A method using Doppler shift by blood flow, which is disclosed in “Optical Coherence Angiography” by Makita et al., (Optics Express, 14 (17), 7821-7840 (2006), and a method using phase variance of an OCT signal by blood flow, which is disclosed in “Mobility and transverse flow visualization using phase variance contrast with spectral domain optical coherence tomography” by Fingler et al., (Optics Express. Vol. 15, No. 20. pp 12637-12653 (2007)) are proposed. Further, “Speckle variance detection of microvasculature using swept-source optical coherence tomography” (Optics Letters Vol. 33, Iss. 13, pp. 1530-1532 (2008)), “Optimized speckle variance OCT imaging of microvasculature” by Mariampillai et al., (Optics Letters 35, 1257-1259 (2010)), and the specification of U.S. Patent Application Publication No. 2014/221827 disclose a method using intensity variance of an OCT signal by blood flow. In addition, the specification of U.S. Pat. No. 8,433,393 proposes, for example, a method using changes of a phase and an intensity of an OCT signal by blood flow.
However, in the aforementioned methods for detecting blood flow, other than the OCT angiography using Doppler shift, noise derived from a motion of an object (bulk motion noise) is generated. This is because in OCT angiography interfering signals are obtained by measuring almost the same portion for a plurality of times, to generate an image, and therefore a measurement value changes regardless of the blood flow when the object moves during image capturing and the change is erroneously detected as the change of the blood flow. For example, in a case of fast movement by involuntary movement of an eyeball, the bulk motion noise appears in an OCT angiography image as partially linear noise in a main scanning direction.
U.S. Patent Application Publication No. 2014/221827 discloses, as a first method for reducing bulk motion noise, a method for dividing a wavelength spectrum of an interfering signal into a plurality of pieces and then performing frequency analysis to thereby reduce the bulk motion noise instead of reducing resolution in a depth direction. Further, as a second method, a method for positioning tomographic images obtained by performing image capturing a plurality of times to thereby reduce the bulk motion noise is disclosed. Disclosed as a third method is a method for computing decorrelation of the tomographic images obtained by performing image capturing a plurality of times in a region at a fixed depth from a retinal surface layer, comparing median values of decorrelation values of decorrelation images for each frame, and removing a frame of an abnormal decorrelation image.
However, the first method of U.S. Patent Application Publication No. 2014/221827 has no effect on the bulk motion noise due to movement of the fundus in a plane direction (directions in a plane perpendicular to a direction of an eye axial length).
Further, with the second method, it is possible to roughly position the tomographic images, but a positional deviation due to distortion of the tomographic images, which is generated during measurement of the tomographic images because of movement of the object for a short period of time is not solved, so that the bulk motion is not reduced in some cases.
In the third method, detection ability is inferior because a part of areas of the tomographic images is used for noise detection. Further, even when the detection is performed successfully, even normal data of the tomographic images having no noise is removed.